1. Field of the Invention
The present invention relates to an implantable medical device of the type having a valve opening detector for detecting opening of a valve at the left side of the heart.
2. Description of the Prior Art
Impedance measurements for detection of the flow mechanical function of a heart are known in the art.
In U.S. Pat. No. 5,792,194 a transvalvular impedance measurement is disclosed. The measurement is made between an atrium and a ventricle electrode of an implanted electro-catheter to provide information indicative of the mechanical state of the heart. The information may be used to control the pacing rate of a rate responsive pacemaker, or the stimulation intensity self adjustment or the pacing mode switching. The maximum measured impedance is indicative of the tricuspid valve (the valve between the right atrium and the right ventricle) being closed and the minimum measured impedance is indicative of the tricuspid valve being open. In the device disclosed in U.S. Pat. No. 5,792,194 a signal generator generates a square wave of frequency 4 kHz, amplitude 3 volts. A voltage level is detected at one of the electrodes inside the heart and the detected signal is amplified in an amplifier having a gain in region 50 to 200 and filtered in a filter network. The filter network uses a notch filter to reject mains frequency (50 or 60 Hz) and also a low pass filter to reject frequencies exceeding 100 Hz, since the frequencies of the signals of interest being well below that level. In that way noise generated in impedance measurement can be reduced.
Impedance measurement using four poles is disclosed in U.S. Pat. No. 4,730,619 wherein an electrode lead adapted to be inserted into a heart is provided with a first and a second ring electrode and a tip electrode. A measurement current at any one of several frequencies is applied between the pacer can and the tip electrode and the voltage measured between the ring electrodes is proportional to impedance. The measured impedance is used to determine changes in volume of blood in the right ventricle and to determine opening and closing of the pulmonary valve to determine ejection time. The pulmonary valve is the valve between the right ventricle and the pulmonary artery.
Impedance measurements within a heart may be used to confirm capture, i.e. to confirm that a stimulation pulse applied to heart tissue has caused a heart contraction. U.S. Pat. Nos. 5,737,883 and 5,902,325 both relate to implantable cardiac stimulators provided with capture detection based upon impedance measurements.
Through impedance measurements, blood volume changes are detectable within the area of measurement. Blood has higher conductivity (lower impedance) than myocardial tissue and lungs. The impedance-volume relationship is inverse; the more blood—the smaller the impedance.
In a well functioning heart the left side and the right side of the heart contract more or less simultaneously starting with the contraction of the atria flushing down the blood through the valves separating the atria from the ventricles. In the right side of the heart through the tricuspid valve and in the left side of the heart through the mitral valve. Shortly after the atria contraction the ventricles contract resulting in increasing blood pressure inside the ventricles that first closes the one way valves to the atria and after that forces the outflow valves to open. In the right side of the heart it is the pulmonary valve, that separates the right ventricle from the pulmonary artery that leads blood to the lung to be oxygenated, which is opened. The right side of the heart is termed the low pressure side of the heart since the pressure is several times lower than the pressure in the left side of the heart-termed the high pressure side. In the left side of the heart the aortic valve separates the left ventricle from the aorta that transports blood to the whole body. The outflow valves, the pulmonary valve and the aortic valve, open when the pressure inside the ventricles exceeds the pressure in the pulmonary artery and the aorta, respectively. The ventricles are separated by the intraventricular elastic septum.
The heart is supplied with blood via the left and right coronary arteries having openings close to the aortic valve. The venous blood is returned to the right atrium by the great cardiac vein to the coronary sinus having its opening close to the inferior vena cava.
The measurement device disclosed in U.S. Pat. No. 5,792,194 is adapted to perform measurements in the right side of the heart, the low pressure side of the heart. There is a great interest in making measurements reflecting the hemodynamic of the left side of the heart in order to increase the possibilities to correctly analyze the function of the heart.